Characterization of a Triplet Vinylidene

Abstract

Singlet vinylidenes (R 2 C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters ( D = 0.377 cm –1 and | E| / D = 0.028) were determined, and the 13 C hyperfine coupling tensor was obtained by 13 C-ENDOR measurements. Most strikingly, the isotropic 13 C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Recent advances of mesoionic N-heterocyclic olefins

“…Perspective The diazoolefins 8 and 9 can liberate dinitrogen upon photolysis to give vinylidenes 27 and 28 with a triplet ground state (Scheme 5). 44,50 The electronic structure of vinylidene 28 was thoroughly studied by EPR/ENDOR spectroscopy and computation. 50 The zero-field splitting parameters (D = 0.377 cm −1 and |E|/D = 0.028) fall in the range for triplet carbenes (D = 0.346-0.409 cm −1 ).…”

“…44,50 The electronic structure of vinylidene 28 was thoroughly studied by EPR/ENDOR spectroscopy and computation. 50 The zero-field splitting parameters (D = 0.377 cm −1 and |E|/D = 0.028) fall in the range for triplet carbenes (D = 0.346-0.409 cm −1 ). 51 In contrast, the 13 C hyperfine coupling constant (50 MHz) obtained by 13 C-ENDOR measurements is far smaller than those of triplet carbenes (175-260 MHz), [52][53][54] suggesting a unique electronic structure.…”

“…The C α -C β bond of the computed structure of 28 displays characteristics (length 1.39 Å, Mayer bond order of 1.45) between a typical single and double bond. 50 At ambient temperature, the triplet vinylidenes 27 and 28 undergo rapid insertion of the carbene carbon into the C-H bond of the adjacent phenyl group to give 29 and 30, respectively. 47,50 Organocatalysis Yan and co-workers have demonstrated the catalytic activities of mNHO 6 toward the hydroboration of aldimines, nitriles, and N-heteroarenes with HBpin (Scheme 6).…”

“…50 At ambient temperature, the triplet vinylidenes 27 and 28 undergo rapid insertion of the carbene carbon into the C-H bond of the adjacent phenyl group to give 29 and 30, respectively. 47,50 Organocatalysis Yan and co-workers have demonstrated the catalytic activities of mNHO 6 toward the hydroboration of aldimines, nitriles, and N-heteroarenes with HBpin (Scheme 6). 43 Compared to IPrCH 2 (IPrCH 2 = 1,3-bis(2,6-diisopropylphenyl)-2-methylene-2,3-dihydro-1H-imidazole), mNHO 6 gives higher yields at lower catalyst loadings.…”

Vanadium complexes with N-heterocyclic vinylidene ligands

“…Theoretical 30,32 and experimental studies 33 have provided evidence that N-heterocyclic vinylidenes display a triplet ground state. In analogy to Schrock-type carbenes, with alkylidene ligands in a triplet ground state, 34 we assumed that early transition metals would be suited for stabilizing N-heterocyclic vinylidenes.…”